Color-size Relations Of Disc Galaxies With Similar Stellar Mass

We select 7959 face-on disc galaxies form SDSS DR7 main galaxy sample as our sample,and investigate the relations between optical colors and size of disc galaxies with similar luminosity or stellar mass.More luminous galaxies are redder and have bigger sizes. In order to get the color-size relations of disc galaxies with similar luminosity, we define two parameters,"relative size"and"relative color". The relative size and the relative colors are defined as the residual of sizes on size- magnitude relation and the residuals of colors on color- magnitude relations, respectively. Colors we used are u-r, g-r, r-i and r-z of galaxies in our sample. The relative size and the relative colors of one galaxy in our sample are defined to measure relative size and colors of the galaxy compared with the mean size and colors of galaxies which are with similar luminosity as the galaxy.We investigate the relations between the relative size and the relative colors of galaxies in our sample. We find under the condition galaxies have similar luminosity, u-r increase with size; g-r is independent on size; r-i and r-z decrease with size. Bigger galaxies have redder u-r color, bluer r-i and r-z color, size is independent on g-r color. We also find the relation between u-r color and size of equal-luminousity galaxies is mainly contributed by less luminous galaxies.We also investigate the relations between optical colors and size of disc galaxies under the condition that galaxies have similar stellar mass: We find galaxies with bigger size have bluer g-r,r-i and r-z color, the size is independent on u-r color. In order to make sure the relations we find are not driven by the color-stellar mass relation and the size-stellar mass relation. We construct"an artificial sample". Galaxies'colors and sizes in the artificial sample are from random assignment, but remain the color -stellar mass relation and the size-stellar mass relation. We find the relations between the relative size and the relative colors of galaxies in the artificial sample are weak. The results of artificial sample show us the reality of the relations we get from the real sample.Considering the colors of galaxies are driven by the stellar population of galaxies. By comparing the observed colors (u-g, g-r, g-i and g-z) of galaxies in our sample with the colors given by the stellar population synthesis, we analyze the stellar population of galaxies in our sample. We find that the massive galaxies are older and have higher metallicity, but the sizes of galaxies are independent on their mean stellar age and metallicity. Only use the optical color can not give us reliable information of stellar population to explain the relations we get from observation. The explanation of the relations between colors and size of mass-equal disc galaxies need further discussion.